The invention relates to an image forming apparatus and an image forming method, and may be suitably applied to an image forming apparatus such as a printer and a copier that use an electro-photography system.
In an existing image forming apparatus using an electro-photography system, an image forming unit that forms a toner image is provided detachably on a main body of the image forming apparatus, and the toner image developed on a photosensitive drum of the image forming unit is transferred to and fixed on a recording medium. In an image forming apparatus that forms a color image, an image forming unit of a spot-color such as white and transparent is detachably provided on a main body of the image forming apparatus, in addition to image forming units of black, yellow, magenta, and cyan, and toner images of the respective colors are overlapped with one another on an intermediate transfer belt to form a color image. Although the plurality of image forming units are normally in contact with the intermediate transfer belt, separating the image forming unit of unused color from the intermediate transfer belt prevents attachment of fogging toner and prolongs lifetime of the image forming unit.
In such an image forming apparatus, a phenomenon called reverse transfer occurs in which a part of the toner transferred from the image forming unit to the intermediate transfer belt is collected by the other image forming unit that is located on the downstream side in a belt traveling direction and is in contact with the intermediate transfer belt. This causes the image density to become lower than a desired value. Thus, an existing image forming apparatus transfers an image density adjusting pattern in response to change in the contact-separation state of the image forming unit to the intermediate transfer belt, and corrects an image forming condition on the basis of a detected image density, thereby offsetting the decrease in image density by the reverse transfer irrespective of the contact-separation state of the image forming unit to obtain appropriate image density. For example, reference is made to Japanese Unexamined Patent Application Publication No. 2011-112797.